In electrical machining of this type, of which a most typical form utilizing a succession of electroereosive electrical discharges is commonly called "traveling-wire EDM" or "wire-cut EDM", the wire electrode is continuously advanced from a supply side, e.g. in the form of a wire-storage reel, to a takeup side, e.g. in the form of a wire-takeup reel, through a machining zone in which the workpiece is positioned. The machining zone is flushed with a machining liquid, typically distilled water or a liquid dielectric (in electrical discharge machining or EDM) or a liquid electrolyte (in electrochemical machining or ECM), or a weakly conductive liquid electrolyte (in electrochemical-discharge machining or ECDM). The workpiece is juxtaposed with the axially traveling wire across the machining zone between a pair of guide members which serve to stretch the traveling wire for positioning it precisely in a predetermined machining relationship with the workpiece. A series of electrical pulses are applied across a machining gap formed between the workpiece and the traveling wire electrode to effect time-spaced electrical discharges thereat to electroerosively remove material from the workpiece in the typical EDM mode of wire-cutting process. In the ECDM mode of wire-cutting process, electrolytic material solubilization is also utilized in addition to EDM action. The ECM mode of operation utilizes a purely electrolytic action for the workpiece material removal process and may make use of a continuous DC current.
As material removal proceeds, the workpiece is displaced relative to and transversely to the continuously traveling wire electrode, typically under numerical control, along a predetermined path to generate a desired pattern of cut in the workpiece. The continuous advancement or travel of the wire is effected typically by traction drive rollers disposed at a location between the guide member on the downstream side and the wire takeup means. A desired tension is established in the traveling wire typically by providing brake rollers at a location between the guide member on the upstream side and the wire supply means.
It is known that the achievement of a satisfactory machining accuracy requires the use of a wire electrode as thin as 0.05 to 0.5 mm in diameter. The machining liquid is supplied to the machining gap to serve as an electrical machining medium, to carry away the machined products and simultaneously to cool the thin electrode and the workpiece.
The present invention is based upon the recognition that in the traveling-wire electroerosive process with these unique characteristics, machining instability is created from time to time. It has now been recognized that machining instability is brought about as a function of the shape of the cutting path and especially in the region of turning points in the path traveled by the wire electrode. When machining becomes unstable, machining discharges tend to shift into an arc discharge or short-circuiting type electrical current passage, resulting in excessive heating of the wire electrode and consequently the breakage of the wire electrode and a reduced machining accuracy and efficiency.